The control of friction and wear of metal mechanical components that are in sliding or rolling-sliding contact is of great importance in the design and operation of many machines and mechanical systems. For example, many steel-rail and steel-wheel transportation systems including freight, passenger and mass transit systems suffer from the emission of high noise levels and extensive wear of mechanical components such as wheels, rails and other rail components such as ties. The origin of such noise emissions, and the wear of mechanical components may be directly attributed to the frictional forces and behaviour that are generated between the wheel and the rail during operation of the system.
In a dynamic system wherein a wheel rolls on a rail, there is a constantly moving zone of contact. For purposes of discussion and analysis, it is convenient to treat the zone of contact as stationary while the rail and wheel move through the zone of contact. When the wheel moves through the zone of contact in exactly the same direction as the rail, the wheel is in an optimum state of rolling contact over the rail. In so such a case, no appreciable friction exists between the wheel and the rail. However, because the wheel and the rail are profiled, often misaligned and subject to motions other than strict rolling, the respective velocities at which the wheel and the rail move through the zone of contact are not always the same. This is often observed when fixed-axle railcars negotiate curves wherein true rolling contact can only be maintained on both rails if the inner and the outer wheels rotate at different peripheral speeds. This is not possible on most fixed-axle railcars. Thus, under such conditions, the wheels undergo a combined rolling and sliding movement relative to the rails. Sliding movement may also arise when traction is lost on inclines thereby causing the driving wheels to slip.
The magnitude of the sliding movement is roughly dependent on the difference, expressed as a percentage, between the rail and wheel velocities at the point of contact. This percentage difference is termed creepage.
At creepage levels larger than about 1%, appreciable frictional forces are generated due to sliding, and these frictional forces result in noise and wear of components (H. Harrison, T. McCanney and J. Cotter (2000), Recent Developments in COF Measurements at the Rail/Wheel Interface, Proceedings The 5th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems CM 2000 (SEIKEN Symposium No. 27), pp. 30-34, which is incorporated herein by reference). The noise emission is a result of a negative friction characteristic that is present between the wheel and the rail system. A negative friction characteristic is one wherein friction between the wheel and rail generally decreases as the creepage of the system increases in the region where the creep curve is saturated. Theoretically, noise and wear levels on wheel-rail systems may be reduced or eliminated by making the mechanical system very rigid, reducing the frictional forces between moving components to very low levels or by changing the friction characteristic from a negative to a positive one, that is by increasing friction between the rail and wheel in the region where the creep curve is saturated. Unfortunately, it is often impossible to impart greater rigidity to a mechanical system, such as in the case of a wheel and rail systems used by most trains. Alternatively, reducing the frictional forces between the wheel and the rail may greatly hamper adhesion and braking and is not always suitable for rail applications. In many situations, imparting a positive frictional characteristic between the wheel and rail is effective in reducing noise levels and wear of components.
It is also known that, wear of train wheels and rails may be accentuated by persistent to and fro movement resulting from the presence of clearances necessary to enable a train to move over a track. These effects may produce undulatory wave patterns on rail surfaces and termed corrugations. Corrugations increase noise levels beyond those for smooth rail-wheel interfaces and ultimately the problem can only be cured by grinding or machining the rail and wheel surfaces. This is both time consuming and expensive.
There are a number of lubricants known in the art and some of these are designed to reduce rail and wheel wear on rail roads and rapid transit systems. For example, U.S. Pat. No. 4,915,856 discloses a solid anti-wear, anti-friction lubricant. The product is a combination of anti-wear and anti-friction agents suspended in a solid polymeric carrier for application to the top of a rail. Friction of the carrier against the wheel activates the anti-wear and anti-friction agents. However, this product exhibits poor retentivity under field conditions and must be reapplied at frequent intervals.
U.S. Pat. No. 5,308,516, U.S. Pat. No. 5,173,204 and WO 90/15123 relate to solid friction modifier compositions having high and positive friction characteristics. These compositions display increased friction as a function of creepage, and comprise resins to impart the solid consistency of these formulations. The resins employed included amine and polyamide epoxy resins, polyurethane, polyester, polyethylene or polypropylene resins. European Patent application 0 372 559 relates to solid coating compositions for lubrication which are capable of providing an optimum friction coefficient to places where it is applied, and at the same time are capable of lowering abrasion loss.
While a number of solid stick compositions in the prior art exhibit a range of friction characteristics, including lubricant compositions or compositions with a positive friction characteristic, a limitation of these solid stick compositions is their fast consumption rate. Solid stick compositions must be repeatedly applied to the rail head or flange interface to ensure proper efficacy and such repeated application can result in substantial costs due to fast consumption rates of the sticks. Thus, there is a need for solid stick compositions which exhibit decreased consumption and wear to prolong the life of the sticks, while still provided effective control of friction and wear of metal mechanical components that are in sliding or rolling-sliding contact. Such solid stick compositions may be effectively used in either closed or open rail systems.